Subtract $ \dfrac{5}{3x^2} $ from $ \dfrac{4}{5x} $ to get $ \dfrac{ \color{purple}{ 12x^2-25x } }{ 15x^3 }$.

To subtract raitonal expressions, both fractions must have the same denominator.
We can create a common denominator by multiplying the first fraction by $ \color{blue}{ 3x^2 }$ and the second by $\color{blue}{ 5x }$.

$$ \begin{aligned} \frac{4}{5x} - \frac{5}{3x^2} & = \frac{ 4 \cdot \color{blue}{ 3x^2 }}{ 5x \cdot \color{blue}{ 3x^2 }} - \frac{ 5 \cdot \color{blue}{ 5x }}{ 3x^2 \cdot \color{blue}{ 5x }} = \\[1ex] &=\frac{ \color{purple}{ 12x^2 } }{ 15x^3 } - \frac{ \color{purple}{ 25x } }{ 15x^3 }=\frac{ \color{purple}{ 12x^2-25x } }{ 15x^3 } \end{aligned} $$